Same reason your government doesn't set taxes to 100%, has rules for themselves they at least sort of follow, and only treats people like shit when they've got a pretext. If they make it "too bad" then they'll have to piss away a whole lot more money on ensuring compliance, it'll threaten stability, etc.

Slaves are the least motivated and least productive form of workers. Slaves who know they're slaves are worse still. Shooting for maximum extraction of labor doesn't actually get you the best ROI. Don't get me wrong, they'll still treat you like shit. But they maximize their "take home" by not going too far with it.

I get that we all want to turn off our brains and hand wring because "criminals" or whatever but the dynamics of human organizations are unchanged regardless of what side of the law they're operating on.